Broad band transmission system



Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE BROAD BAND TRANSIVIISSION SYSTEM Harvey H. Benning, Lincoln Park, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 22, 1938, Serial No. 203,604

10 Claims. ((31.178-44) This invention relates to transmission circuits For a more complete understanding of the inand more particularly to a broad band system vention, reference is made to the drawing in employing a plurality of one-way repeaters. which:

It is the object of the invention to provide Fig. 1 shows in simplified diagram a single 6 means whereby an active repeater may be rerepeater in service; placed without circuit disturbance. Fig. 2 is a schematic of an active and a spare In a broad band transmission system it is the repeater in condition to be interchanged; practice to use a large number of repeaters Fig. 3 shows a schematic of connection after spaced at rather close intervals. In general, these a spare repeater has been substituted for an acrepeaters transmit in one direction only, an additive repeater; and 10 tional circuit being provided for the transmis- Fig. 4 is a general layout of the arrangement sion in the opposite direction. With a large of apparatus for making repeater substitution. number of repeaters thus connected in tandem In Figs. 1, 2 and 3 transmission circuits are it is necessary to hold the amplification produced Shown by a single line for the sake of simplic l by any one repeater within very close limits. A In Fig. 1 a repeater receives energy from line 15 satisfactory method of accomplishing this result fi s it and transmits t to line is provided by the pilot frequency regulator, Th The regulator 2| is shown connected across the pilot frequency may be any convenient frequency repeater at its output and it is presumed that Within the band of transmission of the system. under the conditions of normal operation the 20 The regulator may consist of a form of ampliregulator has set the gain of the repeater at a 20 tier-rectifier bridged across a repeater output. posit o dway between um a d a As it will usually be sensitive only to the pilot mum. A ordin to th convention ad p d in frequency it may provide a direct current pothe drawing this is indicated by the indicator 2'6. tential which is proportional to the level of the It should be understood that this convention has pilot frequency at the repeater output which been adopted in the drawing to aid in explain- 25 potential, being (in one type of regulator) aping the operation of the invention. It has no plied to the grid of a variable mu tube which counterpart in the actual repeater and under forms a part of the amplification system of the ordinary circumstances it would not be possible repeater, will serve to increase or decrease the to determine by in p What gain Was be n 0 gain of the repeater if required until the level produced by any particular repeater. The pilot of the pilot frequency transmission at the revoltmeter of Fig. lmight consist of an amplifierpeater output shall reach its designed value. rectifier with input tuned to the pilot frequency Thus the output level of the repeater as judged and having an output meter calibrated to indiby the pilot frequency is held within very close cate the pilot fr qu y v ta t the n p s limits. tion at which the pilot voltmeter may be con- 35 A broad band system such as that under connected ina manner known in the art. sideration will ordinarily carrya very large num- Referring now to Fig. 4; the normal active ber of channels and it is, therefore, of great imrepeater l, equipped with regulator 3 under norportance that no interruption of service over it mal operating conditions is connected directly beoccurs. Particularly, means must be provided for tween line in and line out, the patching cord 40 the adequate maintenance of repeaters without l'l being used to bridge jack 9 to jack l0 and appreciable interruption or change in transmisthe patching cord l8 being used to bridge jack l4 sion characteristic. It is proposed to provide to jack 13. Under the conditions shown in Fig. spare repeaters which may be quickly substituted 4, however, it is assumed to have been decided for those in active service but due to the feature that repeater I should be removed from the line 45" of automatic gain control the spare repeaters will for inspection. Without removing patching cords ordinarily not immediately provide the proper IT and I8 the spare repeater circuit is set up. output level. Ina specific disclosure of my in- This circuit may be traced from jack II which vention as described below, I provide a method is in multiple with jack 9 through the variable 5 for insuring against this contingency. This disloss equalizer 5, thence through normally closed closure is based upon the presumption, which contacts of the quick-acting switch l9 back to is a matter of properly controlled design and jack [2 which is in multiple with jack l0, also .manufacture, that the amplification of all refrom jack I6 in multiple with jack l4 through peaters at maximum gain setting will be identinormally closed contacts of the quick-acting cal. switch and back to jack l5 which is in multiple 55 with jack I3. The variable loss equalizer is a network which has been designed to have the same transmission frequency characteristic as the actual line and to be variable in attenuation upwards from zero. It is assumed that under the conditions just described, that is, with patching cords I1 and I8 still in place, the variable loss equalizer has been set for zero attenuation. It will be noted thatthe pad" 6 is shown bridged across the strap between jacks I6 and I5. This is assumed to be a bridging pad whose impedance is so high as to cause no appreciable attenuation" or distortion when bridged across a transmission line such as that under consideration. It will now be apparent that with the variable loss equalizer 5 set at zero andwith the pad G assumed to cause no appreciable loss or distortion, the patching cords I1 and I8 may be removed, so long as the quick-acting switch I9 is in the position shown, without any disturbance to" the line circuit. The conditionsshown in Fig. 4 are equivalent to those oi'Fig. 1 so far as line transmission is concerned. There is in Fig. 4, however, an auxiliary circuit which may be traced from the output of repeater I through pad 6, normally closed contacts of switch I9, and thence to the input of repeater Zwhich is equipped with regulator 4. The output of repeater 2 is traced through normally closed contacts of switch I9 to network I. The function of high impedance bridging pad 6 is two-fold. It not only picks pilot frequency off the line for transmission to the input of repeaterl but it also so attenuates the level of that pilot frequency that thepilot frequency levelafter aanplificationby repeater 2 isstill well below the levelwhich repeater 2 and regulator 4 are designed to maintain. In the effort to achieve this designed level, therefore, the regulator 4 steps up repeater 2 to its maximum gainr 7 Pilot voltmeter 21 may then be employed to check-the satisfactory operationof repeater 2. It is apparent thatrepeaterl may be a regulator controlled repeater equivalent to repeater I or it may be any suitable repeater set to have a gain equivalent to the maximum gain of repeater I.

Returning now to the main transmission circuit, in the condition shown in Fig. 4, with patching cords I I and. I8 removed, if attenuation is slowly cut into the variable loss equalizer 5thus decreasing the level of pilot frequency and, of course, all other transmitted frequencies: into repeater I, regulator-3 will step up the gain of repeater I thus keeping the pilot frequency'output level constant. This operation is continued until repeater I reaches the point of maximum gain as evidenced by the fact that for a further small increase of attenuation the voltage as measured by thepilot voltmeter will decrease slightly. The condition of Fig. 2 has now been reachedin which both active and spare repeaters are set at maximum gain and are, therefore, set for equal amplifications If now the quick-acting switch I9 is thrown to its alternate position the conditions become those of Fig. 3 in which 2 is now the active repeater and I is out of active service and may be serviced or replaced without disturbance to the line transmission system. When service of repeater I has been completed the quick-acting switch I9 maybe thrown back toits normal position as shown in Fig. 4. The variable equalizer is then gradually cut down to zero, the patching cords I1 and I8 are replaced, and normal operating conditions are thus restored;

It is to he -understood that the various struts-- tures and circuit arrangements are merely illustrative and that many modifications may be made without departing from the spirit of the invention as defined by the claims.

What is claimed is:

1. In a transmission system, a transmission line, a repeater having an input and an output connected in said line, a spare repeater, an automatic gain regulator for each'repeater adapted to be controlled in accordance with current on said line for adjusting the gain setting of said repeaters, means to connect the input of said spare repeater to receive highly attenuated currents from said" line, whereby its regulator is caused to adjust its gain to maximum setting, means for attenuating the current to said repeater in said line to cause its regulator to adjust its gain to maximum setting, and a switch for substituting said'spare repeater in place of saidrepeater in said line.

2. The method of servicing a repeater in a transmission line self-regulated by line current which consists in attenuating the line current at the repeaterinput to cause said line repeater to regulateits gain to maximum, impressing highly attenuated line current-on a spare repeater to cause said spare repeater to regulate its gain to maximum and-substituting said spare repeater for said line repeater.

3(The method of making a service replace-. ment of a self-regulated repeater in a transmis-' sion line which comprises increasing the attenuation of said lineto the point at which the said repeater is self-regulated to a condition of maximum gain, and quickly substituting a spare repeater preadjusted to the same gain for said -repeater in said transmission line.

4. The method of replacing in a transmission line, an active line repeater having an automatic gain regulator adapted to becontrolled in accordance with current in said line by a similarly controlled spare repeater, comprising the introduction ofattenuation into said transmission line to cause said active repeater to be regulated to maximum gain setting, the eduction of highly attenuated currents from said transmission line to cause said spare repeater to be regulated to maximum gain setting and rapid substitution of said spare repeater for said active repeater.

5. In a transmission system, a transmission line, an amplifier in said line, a spare amplifier arranged to receive input current from said line and having its output connected to a dummy load, means for adjusting said line amplifier to a predetermined gain above the normal gain of said line amplifier without change in net system gain means for adjusting said spare amplifier to said predetermined gain, and a multicontact switch adapted to interchange respectively the inputs of said amplifiers and simultaneously to interchange respectively the outputs of said amplifiers.

6. In a transmission system employing self-adjustable amplifiers having a margin of available gain above normal operating gain, replacement means for an active amplifier comprising in combination an equivalent spare amplifier, means for causing said active amplifier to cancel its normal operating margin so as to adjust its gain to maximum while maintaining constant system gain, auxiliary means causing said spare amplifier to regulate its gain intoequivalence with said active amplifier, and a quick-acting switch adapted to substitute said spare amplifier for said active amplifier.

7. In a transmission system employing constant output amplifiers, replacement means for an active amplifier comprising in combination an equivalent spare amplifier, means including a variable attenuator for causing said active amplifier to adjust its gain to a maximum while maintaining constant system gain, auxiliary means including an input loss pad and a terminating network causing said spare amplifier to regulate its gain into equivalence with said active amplifier, and a quick-acting switch adapted to interchange said amplifiers.

8. In a transmission system designed to maintain constant output level by the employment of self-regulatory amplifiers, replacement means for an active amplifier comprising a similar spare amplifier, means for causing said active amplifier to adjust its gain to a predetermined level, means for causing said spare amplifier to adjust its gain to a level equal to that of said active amplifier, and a hand operated switch for interchanging said amplifiers.

9. In a transmission system arranged to automatically maintain a constant output level by the employment of self-adjustable amplifiers, re-

placement means for an active amplifier comprising a similar spare amplifier, means for causing said active amplifier to increase its gain to a predetermined level without change in system level, independent means causing said spare amplifier to adjust its gain to equivalence with said active amplifier, and a quick-acting switch for interchanging said amplifiers.

10. In a transmission system designed to automatically compensate for changes in system attenuation by the employment of self-adjustable amplifiers, replacement means for an active amplifier comprising a spare amplifier of identical design, hand operated means for increasing the system attenuation preceding said active amplifier whereby it is caused to increase its gain to maximum without change in the transmission equivalent of the said system, input means causing said spare amplifier to adjust its gain to maximum, and a switch adapted to quickly interchange the input and the output connections respectively of said amplifiers.

HARVEY H. BENNTNG. 

